Oxygen isotopes in cellulose identify source water for archaeological maize in the American Southwest

Maize (Zea mays) was a primary food crop for aboriginal societies of the arid American Southwest. Water used for maize production in these arid zones could have come from precipitation and runoff during the summer monsoon, from perennial streams and springs, or from stored soil water fed by snowmelt. The oxygen stable isotope ratio (δ18O) of summer and winter precipitation on the Colorado Plateau naturally differ by more than 10‰ providing a powerful tool for distinguishing winter- from summer-derived water sources used in cultivation of maize. We investigated whether variation in δ18O of potential source waters is preserved in the δ18O of cellulose (δ18Ocellulose) of maize cobs by growing four aboriginal and one modern maize variety in pots irrigated with water of known δ18O composition. The δ18Ocellulose values of cobs ranged from 26.8 to 36.4‰ (averaged within varieties) and were highly correlated with δ18O values of the source irrigation water (−15.8 to −8.2‰). Cob δ18Ocellulose from five archaeological sites on the Colorado Plateau in southeastern Utah ranged from 27.3 to 34.6‰, closely matching the range of values observed in experimental plants. A δ18Ocellulose model developed originally for tree rings was parameterized and applied to the archaeological maize cobs. The model indicated that monsoonal precipitation accounted for 0-20% of the moisture for archaeological cob samples from a site adjacent to a perennial stream and 43-98% for samples from an upland site, more distant from a perennial water source. These results reveal the potential for using δ18Ocellulose to investigate prehistoric irrigation practices and source water used for maize production in the American Southwest.